As compared with widely used amorphous silicon (a-Si), an amorphous (noncrystalline) oxide semiconductor has high carrier mobility (which may be referred to as electron field-effect mobility, hereinafter may be often referred to as simply “mobility”), a high optical band gap, and film formability at low temperature and, therefore, has been highly expected to be applied for next generation displays which are required to have a large size, high resolution, and high-speed drive, resin substrates which have low heat resistance, and the like (Patent-Document 1 and the like).
Of oxide semiconductors, an amorphous oxide semiconductor containing indium, gallium, zinc, and oxygen (In—Ga—Zn—O, hereinafter also referred to as “IGZO”), which has a considerably high carrier mobility, is particularly preferably used. For example, Non-Patent Documents 1 and 2 disclose a thin film transistor (TFT) including a thin oxide semiconductor film of In:Ga:Zn=1.1:1.1:0.9 (atomic % ratio) as a semiconductor layer (active layer).
In the case where an oxide semiconductor is used as a semiconductor layer for a thin film transistor, the oxide semiconductor is required not only to have a high carrier concentration (mobility) but also to be excellent in switching properties (transistor characteristics, TFT characteristics) of TFT. Specifically, the oxide semiconductor is required to satisfy (1) high ON-current (maximum drain current when positive voltage is applied to a gate electrode and a drain electrode); (2) low OFF-current (drain current when negative voltage is applied to a gate electrode and positive voltage is applied to a drain electrode); (3) low SS value (Subthreshold Swing, gate voltage required to increase drain current by one digit); (4) stability of threshold voltage with the lapse of time (voltage at which drain current starts flowing when positive voltage is applied to a drain electrode and either positive or negative voltage is applied to a gate voltage); (5) a high mobility, and the like.
Furthermore, a TFT using an oxide semiconductor layer such as IGZO and the like is required to be excellent in resistance to stress (stress stability) of voltage application, light irradiation, and the like. For example, when voltage is continuously applied to a gate electrode or when a gate electrode is continuously irradiated with light in a blue emitting band in which light absorption starts in a semiconductor layer, charge is trapped in the interface of the semiconductor layer with a gate insulator layer of the thin film transistor, and the threshold voltage is considerably changed (shifted) to a negative side due to a change of charge inside the thin film transistor, and it is pointed out that because of that, the switching properties of the TFT are changed. The change of the switching properties due to the stress caused by light irradiation or voltage application leads to lowering of reliability in a display device itself.
Furthermore, similarly in an organic EL display, the semiconductor layer is irradiated with light leaked out from a light emitting layer, and the problem in which a value such as a threshold voltage varies may cause.
As described above, the shift of threshold voltage particularly leads to lowering of reliability in a display device itself such as a liquid crystal display or an organic EL display equipped with a TFT, and, therefore, it has been extremely desired to improve the stress stability (small change before and after stress test).
For example, Patent Document 2 proposes a technology of improving the electric characteristics of a TFT. Patent Document 2 discloses a technology in which the hydrogen concentration of an insulator layer (including a gate insulator layer) in contact with an oxide semiconductor layer forming a channel region is reduced to less than 6×1020 atoms/cm3 to suppress diffusion of hydrogen into the oxide semiconductor layer. If hydrogen is diffused into the oxide semiconductor layer, the excessive carriers are generated in the oxide semiconductor layer. Thus, the threshold voltage shifts in the negative direction, and drain current flows even in the state (Vg=0V) where voltage is not applied to the gate electrode (normally-on), and therefore the electric characteristics of the transistor may be degraded. Under such circumstances, it is described in Patent Document 2 that the diffusion of hydrogen into the oxide semiconductor layer is suppressed by allowing the insulator layer in contact with the oxide semiconductor layer to be an oxide insulator layer having a reduced hydrogen concentration, and oxygen is supplied to defects of the oxide semiconductor layer from the insulator layer, thereby improving the electric characteristics of the transistor. In Patent Document 2, it is described that in order to exhibit such an effect, it is necessary that the hydrogen concentration in the insulator layer be reduced to less than 6×1020 atoms/cm3. Moreover, it is also described that when such an insulator layer having a reduced hydrogen concentration is formed by a plasma CVD method, a gas in which hydrogen is not contained in its molecular structure is needed to be selected and used as the deposition gas (that is, not SiH4 usually used but SiF4 is used). However, the above Patent Document 2 pays no attention to an improvement in stress stability (particularly, a decrease in change of the threshold voltage against light and a bias stress).